JPS6023303B2 - Method of measuring susceptibility to intergranular corrosion and stress corrosion cracking - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for non-destructively measuring the intergranular corrosion and stress corrosion cracking susceptibility of austenitic nickel alloys.

Austenitic nickel alloys, such as N1oy60
0,690 etc. contain large amounts of nickel (Ni) and chromium (Cr) to increase corrosion resistance and ensure high temperature strength.
and other small amounts of elements.

Austenitic nickel alloys composed of such components become sensitized by heat treatment, welding, or the effects of high-temperature use environments, resulting in increased susceptibility to intergranular corrosion and stress corrosion cracking. If susceptibility to such corrosion and cracking could be measured nondestructively, it would be extremely convenient for evaluating the quality of alloy products before use and for predicting corrosion resistance during use. In particular, it will be possible to evaluate the corrosion resistance of welded parts after equipment is assembled, and to predict changes in the corrosion resistance of parts during operation before corrosion or cracks actually occur, which will greatly contribute to ensuring equipment safety. . However, conventional methods for measuring the sensitization of metals include the nitric acid test and the U-Pend test.

In the nitric acid test, a part of the material to be measured is taken as a test piece, and the test piece is immersed in a boiling nitric acid aqueous solution to measure the amount of intergranular corrosion. On the other hand, in the U-pend test, a part of the material to be measured is similarly taken as a test piece, this test piece is bent into a U shape and immersed in a certain corrosive environment, and the presence or absence and length of cracks in the test piece are determined. Investigate and measure stress corrosion cracking.
Both of these conventional measurement methods are destructive tests. This method of measuring secondary sensitization, which requires the collection of test pieces, is limited in its scope of use to objects to be measured from which test pieces can be collected. For example, it cannot be used for finished products, equipment, or processed parts such as welded parts, and it has been impossible to continuously check the aging changes in sensitization during use of metal products. Furthermore, due to the nature of the test, this conventional measurement method requires a large amount of money, equipment, labor, and time for measurement, and furthermore, the values obtained from the test results have considerable dispersion. . Therefore, an object of the present invention is to provide a measuring method that can accurately measure the intergranular corrosion and stress corrosion cracking susceptibility of an austenitic nickel alloy in a short time without destroying the material to be measured. be.

The present invention will be explained below with reference to the drawings.

Figure 1 shows an example of an austenitic nickel alloy.

N1oy600 (Ni75.0%, Cr15.7%, F
e8.1%, CO. 012%, Si0.33%, Mn0
．． 30%), the change in magnetic permeability when sensitized at 700oo is shown. Intergranular corrosion tests and stress corrosion cracking tests show that this alloy becomes more sensitized by heat treatment at 70,000 °C for up to 300 minutes, after which recovery begins. On the other hand, the magnetic permeability increases as sensitization increases, as shown in FIG. 1, and decreases when recovery begins.
The change in magnetic permeability thus corresponds well with the sensitization and recovery behavior. Therefore, the inventor of the present application predicted that there would be some kind of relationship between the degree of sensitization of an austenitic nickel alloy and the value of magnetic permeability of that alloy, and measured it and obtained the graph shown in FIG. 2.

That is, FIG. 2 shows Anoy600 (Ni75.2%, C
r15.6%, Fe8.0%, CO. 018%,Sio
．． 31%, Mno. 29%), the relationship between the magnetic permeability after various heat treatments and the amount of intergranular corrosion in a nitric acid test (immersion in a boiling 40% nitric acid aqueous solution for 2 minutes) is shown.

This figure shows that there is a strong correlation between the magnetic permeability and the amount of intergranular corrosion when the degree of sensitization of the LOY600 is changed by heat treatment. This is because Cr-Fe carbide precipitates as it becomes sensitized, and Cr-Fe carbide precipitates near the carbide.
This is considered to be due to the fact that a region in which the amount of Nj decreases while the amount of Nj relatively increases is generated, causing magnetic transformation in this region from paramagnetic to ferromagnetic, resulting in an increase in magnetic permeability. The change in magnetism caused by the change in the amount of Cr or Ni is clear from the results shown in FIG. Figure 3 shows Ni
This figure shows the relationship between changes in the Cr group and changes in magnetic permeability in a -Cr-10%Fe alloy. When Cr becomes 10% or less, the magnetic permeability of this alloy suddenly increases and it changes to ferromagnetism. It was also found that there is a strong correlation between not only the amount of intergranular corrosion but also stress corrosion cracking and magnetic permeability. Therefore, what can be seen from FIGS. 1 and 2 is that the susceptibility of an austenitic nickel alloy to intergranular corrosion and stress corrosion cracking can be estimated by measuring its magnetic permeability.

The object to be measured to which this measurement method can be used is an austenitic nickel alloy whose magnetic permeability changes with changes in the amount of Cr due to progress of sensitization, as shown in Figure 3.
It is necessary that the content be approximately 10% or more. Further, when Nj is less than 25%, ferrite or martensite is generated and not only does the austenite phase not occur, but also the austenite does not cause a change in magnetic permeability in response to a change in the amount of Cd. Therefore, the austenitic nickel alloy to which the measurement method of the present invention is applied is an alloy containing 25% or more of Ni and 25% or more of Crlo%.

The susceptibility of such alloys to intergranular corrosion and stress corrosion cracking can be determined by simply measuring the magnetic permeability of the various alloys once the relationship between magnetic permeability and susceptibility has been determined for various alloys according to the present invention. It can be measured non-destructively and easily in a short time. The measurement method according to the present invention makes it possible to measure the susceptibility of finished products, equipment, and processed parts such as welded parts, which was previously impossible, and it is also possible to measure changes in the susceptibility of equipment in corrosive environments over time. This can facilitate inspection and management of equipment for corrosion. Note that changes in magnetism due to sensitization in the above-mentioned austenitic nickel alloys may be measured by measuring permeability, but the relative strength of magnetization, that is, the degree of magnetization in the unsensitized alloy and that after sensitization can be measured. It is also possible by measuring the ratio.

FIG. 4 shows the relationship between the relative strength of magnetization and the amount of intergranular corrosion determined by the nitric acid test for N1oy600. As shown in the figure, there is a strong correlation between the relative strength of magnetization and the amount of intergranular corrosion. Furthermore, it was found that not only the amount of intergranular corrosion but also stress corrosion cracking had a strong correlation with the relative strength of magnetization. Therefore, by determining this correlation in advance for each alloy and then measuring the relative strength of magnetization of completed parts, etc., the susceptibility to intergranular corrosion and stress corrosion cracking can be easily and quickly measured.

[Brief explanation of the drawing]

FIG. 1 shows changes in magnetic permeability due to sensitization treatment in Alloy60. FIG. 2 shows the relationship between the magnetic permeability and the amount of intergranular corrosion in N10y600. FIG. 3 shows the relationship between changes in Cr content and changes in magnetic permeability in an austenitic nickel alloy of Ni-Cr-10%Fe. Figure 4 shows N1oy60
The relationship between the relative strength of magnetization at zero and the amount of intergranular corrosion is shown. Younger brother's figure 2 figure 3 grandchild figure 4

Claims (1)

[Claims] 1. In an austenitic nickel alloy containing Ni: 25% or more and Cr: 10% or more, the change in susceptibility to intergranular corrosion and stress corrosion cracking due to the progress of sensitization and the change in magnetic permeability A method in which a correlation is determined in advance, magnetic permeability is measured, and susceptibility to intergranular corrosion and stress corrosion cracking is determined from the correlation. 2. In an austenitic nickel alloy containing Ni: 25% or more and Cr: 10% or more, the correlation between changes in the susceptibility to intergranular corrosion and stress corrosion cracking due to the progress of sensitization and changes in the relative strength of magnetization was investigated in advance. A method of determining susceptibility to intergranular corrosion and stress corrosion cracking from the above correlation by determining the relative strength of magnetization.